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Data Sheet

LTC9101-2A/LTC9102

APPLICATIONS INFORMATION

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Rev. 0 | 18 of 34

OVERVIEW

Power over Ethernet, or PoE, is a standard protocol for sending DC

power over copper Ethernet data wiring. The IEEE group that ad-

ministers the 802.3 Ethernet data standards added PoE powering

capability in 2003. This original PoE standard, known as 802.3af,

allowed for 48V DC power at up to 13W. 802.3af was widely

popular, but 13W was not adequate for some applications. In 2009,

the IEEE released a new standard, known as 802.3at or PoE+,

increasing the voltage and current requirements to provide 25.5W

of delivered power. IEEE 802.3af and 802.3at are commonly known

as PoE 1. In 2018, the IEEE released the latest PoE standard,

known as 802.3bt or PoE 2. 802.3bt maximizes PD delivered power

at 71.3W.

The IEEE standard also defines PoE terminology. A device that

provides power to the network is known as a PSE, or power sourc-

ing equipment, while a device that draws power from the network

is known as a PD, or powered device. PSEs come in two types:

endpoints (typically network switches or routers), which provide

data and power; and midspans, which provide power but pass

through data. Midspans are typically used to add PoE capability to

existing non-PoE networks. PDs are typically IP phones, wireless

access points, security cameras, and similar devices.

LTC9101-2A/LTC9102 Product Overview

The LTC9101-2A/LTC9102 is a sixth generation PSE controller that

implements up to 48 (25.5W) 2-pair PSE ports in an endpoint

application. Virtually all necessary circuitry is included to implement

an IEEE 802.3at compliant PSE design, requiring an external pow-

er MOSFET and sense resistor per port; these minimize power

loss compared to alternative designs with onboard MOSFETs, and

increase system reliability.

The LTC9101-2A/LTC9102 chipset implements an optional propriet-

ary isolation scheme for inter-chip communication. This architecture

substantially reduces BOM cost by replacing expensive opto-isola-

tors and isolated power supplies with a single low-cost transformer.

A single LTC9101-2A is capable of controlling a bus of up to

four LTC9102s over this transformer-isolated interface. Direct con-

nection of the LTC9101-2A and the associated LTC9102s is also

possible.

The LTC9101-2A/LTC9102 offers a configurable interrupt signal

triggered by per-port events, per-port power on control and fault

telemetry, per-port current monitoring, VEE monitoring, and one

second rolling current and voltage averaging.

The LTC9101-2A/LTC9102 also offers advanced sixth-generation

PSE features including internal eFlash for storage of firmware

updates and custom user configuration packages, I2C quad virtuali-

zation for full backwards-compatibility with quad-based IC drivers,

ultralow 100mΩ sense resistors, +80V/–20V tolerant port-facing

pins, and improvements to cable surge ride through.

Each LTC9102 power channel is implemented with dedicated de-

tection and classification hardware. This allows all ports to detect,

classify, and power on simultaneously, drastically reducing power

on latency across a switch. Other less advanced PSEs are subject

to visible delays as PDs, for example, LED lights, power on a

port-by-port basis.

VEE and port current measurements are performed simultaneously,

enabling coherent and precise per-port power monitoring.

802.3AT 2-PAIR OPERATION

The LTC9101-2A includes up to 12 groups of four identical

ports. Each group of four ports is referred to as a quad. In the

LTC9101-2A architecture, each quad contains register configuration

and status for four 802.3at ports.

The terms port, channel, and power channel are used interchange-

ably throughout this document. These terms refer to a single

802.3at PSE port.

802.3at Type 1 Mode

All 802.3at-compliant PSEs are fully backwards compatible with ex-

isting 802.3at Type 1 PDs, as shown in Table 5. 802.3at-compliant

PSEs are forwards interoperable with new 802.3bt Type 3 and Type

4 PDs.

Note that an 802.3at PSE does not pass an 802.3bt PSE compli-

ance test, and an 802.3bt PSE does not pass an 802.3at PSE

compliance test. This is by design of the respective standards.

802.3at and 802.3bt devices are designed to be interoperable.

Table 5. PSE Maximum Delivered Power, Per-Port

DEVICE

PSE

STANDARD

802.3at

802.3bt

TYPE

802.3at

13W

13W1

25.5W

802.3bt

13W1

25.5W1

51W

13W1

25.5W1

51W1

71.3W

1 Indicates PD allocated less power than requested.

POE BASICS

Common Ethernet data connections consist of two or four twisted

pairs of copper wire (commonly known as Ethernet cable), trans-

former-coupled at each end to avoid ground loops. PoE systems

take advantage of this coupling arrangement by applying voltage

between the center-taps of the data transformers to transmit power

from the PSE to the PD without affecting data transmission. Figure

35 shows a high level PoE system schematic.

To avoid damaging legacy data equipment that does not expect

to see DC voltage, the PoE standard defines a protocol that

determines when the PSE may apply and remove power. Valid

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